This investigation aims at understanding the relationship between metal-ligand electron-nuclear coupling and bond lengths in oxyCo(II) porphyrin complexes, active site models of Co(II)-substituted globins. Co(II)-substituted globins are functional and paramagnetic analogues of O2-carrying hemoproteins. Coupling of the unpaired electron on the oxyCo unit of these proteins and models to an axial cobalt-bound 14N nucleus can be studied by electron spin echo envelope modulation (ESEEM) spectroscopy. The magnitude of the ESEEM-derived 14N electron-nuclear hyperfine coupling and nuclear quadrupole coupling constants have been correlated with the electronic configuration of the trans cobalt-dioxygen bond. Both ligand hyperfine and nuclear quadrupole coupling decrease with an increase in the ionicity of the cobalt-dioxygen bond, i.e., its electronic structure shifts from a Co2+-O2 towards a Co3+-O2- configuration. An increase in the ionicity of the cobalt-oxygen bond would also suggests a shorter bond. Furthermore, a decrease in the axial 14N nuclear quadrupole coupling is indicative of an increase in the overlap between the 14N sp2 hybrid and the Co dz2 orbital and thus implies a decrease in the axial cobalt-nitrogen bond length. The present study therefore aims at testing the hypothesis that a decrease in cobalt-axial nitrogen electron-nuclear coupling is accompanied by a shortening of both the axial cobalt-nitrogen bond and the trans cobalt-oxygen bond using EXAFS spectroscopy. Axial cobalt-ligand bond length measurements are to be carried out in two sets of samples. The first set is oxyCo[tetraphenylporphyrin(TPP)][1-methylimidazole(1-MeIm)] in 100%toluene, 9:1 (v/v) toluene:CH2Cl2 and 1:1 toluene: CH2Cl2. ESEEM studies has found decreases 14N(1-MeIm) electron-nuclear couplings with increasing polarity of the solvent (Aiso from 3.14 to 2.75 MHz, e2qQ from 2.32 to 2.25 MHz). The second set of samples is oxyCo[(o-R)1TPP][1-MeIm] in toluene, where R=H, NHCOC(CH3)3, NHCOCH3 and NHCONHC6H5. The amide proton on R, the para substituent of one of the meso phenyls of TPP, is believed to interact electrostatically with the cobalt-bound axial O2. ESEEM studies has found decreases in 14N(1-MeIm) electron-nuclear couplings with increasing electron-withdrawing strength of R (or increasing acidity of the amide proton) (Aiso from 3.14 to 2.57 MHz, e2qQ from 2.32 to 2.13 MHz). EXAFS spectroscopy is used to test whether shorter cobalt-oxygen and shorter cobalt-nitrogen bonds are found for those complexes with smaller electron-nuclear coupling constants. Using our global mapping technique, we have seen that for the cobalt model compound in toluene, the iron oxygen bond length is 1.90 ( 0.02 , the iron-imidazole bond length is 2.00 ( 0.04 , and the average distance of iron to the equatorial nitrogen bond lengths is 1.96 ( 0.01 . EXAFS studies of the methylene chloride induced structure and other forms with varying kinetics, to better correlates bond lengths and kinetic are continuing.